1. Field of the Invention
The present invention relates to rotation rate sensors.
2. Description of the Prior Art
WO 2005/066585 A1, WO 2005/066584 A1 and U.S. Pat. No. 6,605,164 describe rotation rate sensors (illustrated, for example in FIG. 7) that include an excitation unit 710, a sample mass 730 and a substrate, coupled by spring elements 711, 731 such that the excitation unit 710 can generally move relative to the substrate solely in the direction of a first axis (x axis). A sample mass 730 can move relative to the excitation unit 710 solely in the direction of a second axis (y axis) at right angles to the first axis (x).
Both axes lie on the substrate plane, i.e. the structures do not move at right angles to the substrate beyond manufacturing tolerances. Both force transmitters and taps are not required, by means of which forces can be applied in the z direction and movements can be measured in the z direction. This means that it is also possible to employ manufacturing methods by means of which such z force transmitters and z taps could not be produced.
During operation of the sensor, in the form of a Coriolis gyro, the excitation unit 710 excites a first oscillation in the direction of the first axis (x) (excitation mode). The sample mass 730 is moved (approximately) in this direction with the same amplitude and phase as the excitation unit 710. Force transmitters and taps 714 are provided for the excitation units 710. The excitation mode is excited by the excitation units preferably at its resonant frequency and with a velocity amplitude regulated to a fixed value.
When the Coriolis gyro is rotated about an axis (z) at right angles to the substrate plane, Coriolis forces act on the individual structures in the direction of the second axis (y). Because of the movement degrees of freedom described above, only the sample mass 730 can be deflected by the Coriolis forces. In this case, the sample mass 730 oscillates in the direction of the second axis (y) (referred to in the following text as the detection mode). The amplitude or deflection of the resultant oscillation of the sample mass 730 can be used as a measurement variable. Suitable taps 734, such as electrodes on the sample mass 730 with opposed electrodes anchored on the substrate, are required for this. Alternatively, the Coriolis force can be reset. Force transmitters are required for this. Forces can be applied to the sample mass 730 by the force transmitters (e.g., the electrode arrangement described above, in which the tap and the force transmitter functions can be selectively provided via common electrodes or separate electrodes). The amplitude of the resetting force is then a measure of angular velocity.
The cited documents also describe the ability, in each case, to arrange two of the gyro elements as described above alongside one another on a common substrate and to couple the two drive units and/or the two sample masses with additional spring elements.
WO 02/16871 A1 and U.S. Pat. No. 6,691,571 B2 describe gyros as rotation rate sensors (illustrated in FIG. 8), which have an excitation unit 810, a Coriolis element 820, a detection unit 830 and a substrate coupled by spring elements 811, 821, 831, 832 so that the excitation unit 810 can generally move relative to the substrate only in the direction of a first axis (x axis). The detection unit 830 can move relative to the substrate only in the direction of a second axis (y axis) at right angles to the first axis (x) and the Coriolis element 820 can move relative to the excitation unit 810 only in the direction of the second axis (y) and relative to the detection unit 830 only in the direction of the first axis (x). Both axes lie on the substrate plane, i.e. the structures do not move at right angles to the substrate within manufacturing tolerances. No force transmitters and taps are required, by means of which forces can be applied in the z direction and movements can be measured in the z direction. This means that it is also possible to use such manufacturing methods for production of the structures by which z force transmitters and z taps cannot be produced.
In order to operate the Coriolis gyro, the excitation unit 810 excites a first oscillation in the direction of the first axis (x) (excitation mode). The Coriolis element 820 is moved in this direction (approximately) with the same amplitude and phase as the excitation unit 810. The detection unit 830 is not moved in this approximate direction. Force transmitters and taps 814 are provided for excitation units 810. The excitation mode is excited by the force transmitters preferably at its resonant frequency with a velocity amplitude regulated at a fixed value.
When the Coriolis gyro is rotated about an axis (z axis) at right angles to the substrate plane, Coriolis forces act on moving structures in the direction of the second axis (y). Because of the movement degrees of freedom described above, only the Coriolis element 820 (and not the excitation unit 810) can be deflected by the Coriolis forces, with the detection unit 830 also moved. The Coriolis element 820, together with the detection unit 830, oscillates in the direction of the second axis (y) (referred to in the following text as the detection mode). The amplitude of the resultant detection mode can be used as a measurement variable. Suitable taps 834, such as electrodes on the detection unit 830 with opposed electrodes anchored to the substrate are required for this. The Coriolis force can be reset as an alternative. Force transmitters are required for this. Forces can be applied to the detection mode by the force transmitters (e.g., the electrode arrangement as described, in which the tap and force transmitter functions can be selectively carried out by common electrodes or separate electrodes). The amplitude of the resetting force is then a measure of the angular velocity. WO 02/16871 A1 and U.S. Pat. No. 6,691,571 B2 describe the ability to arrange two of the gyro elements as described above alongside one another on a common substrate, and to couple the two drive units, the two Coriolis elements or the two detection units by additional spring elements.
The sample masses and/or Coriolis elements and detection units are deflected to a relatively major extent by linear acceleration forces in the direction of the second axis, unless these forces are compensated by resetting forces in the prior art. Such “relatively major” deflection is possible due to the fact that the described coupled structures also have eigen modes (referred to in the following text as the “linear mode”), in which the sample masses and/or Coriolis elements and detection units move in phase in the direction of the second axis. The resonant frequency of this linear mode is lower than the resonant frequency w2 of the detection mode. (The expression “relatively major” should be understood to mean that, in the case of a linear steady-state acceleration α in the direction of the second axis, the deflection x is either a few percent:
                    x        =                  α                      ω            2            2                                              (        1        )            or more (ω2 is the resonant frequency of the detection mode). The equation is exact for individual structures. For coupled structures without coupling of the sample masses and/or of the Coriolis elements/detection units, there may be either two independent detection modes, (with somewhat different resonant frequencies due to manufacturing tolerances) and/or weak coupling (e.g., due to the solid structure components having a stiffness which is not infinitely large). This can result in a small amount of common-mode/differential-mode splitting.)
When coupling is intentionally carried out by additional spring elements of the sample masses and/or the Coriolis elements and/or the detection units, the resonance of the detection mode of the individual structures is split into a common mode and a differential mode. The differential mode corresponds to the detection mode, and, in the relevant examples of the prior art, it has a higher resonant frequency than the common mode. The acceleration-dependent deflection is then greater than that given by equation (1).
Without resetting of linear acceleration forces, mentioned above, acceleration-dependent errors occur in the output signal. Such errors are compensated, although only partially, in the case of two coupled gyro units. Resetting reduces error signals, but requires appropriately designed force transmitters. In the case of electrostatic force transmitters, for example, the required size of the electrodes and/or the magnitude of the electrical voltages may have a disadvantageous effect on the mechanical characteristics of the sensor element and/or on the electronics (number of components, power loss, physical size).
Micromechanical gyro structures that have two coupled sample masses and in which the excitation and detection modes each correspond to a linear out-of-phase oscillation of the two sample masses on the substrate plane are known from M. F. Zaman, A. Sharma, and F. Ayazi, “High Performance Matched-Mode Tuning Fork Gyroscope”, Proc. IEEE Micro Electromechanical Systems Workshop (MEMS 2006), Istanbul, Turkey, January 2006, pp. 66-69. The resonant frequencies of the detection and linear modes are identical to a first approximation, (i.e., linear accelerations cause relatively major deflection of the sample mass, leading to error signals).
Micromechanical gyro structures are known in which a detection mode responds to rotary oscillation. See, for example, P. Greiff, B. Boxenhorn, T. King, and L. Niles, “Silicon Monolithic Micromechanical Gyroscope”, Tech. Digest, 6th Int. Conf. on Solid-State Sensors and Actuators (Transducers '91), San Francisco, Calif., USA, June 1991, pp. 966-968, and J. Bernstein, S. Cho, A. T. King, A. Kourepins, P. Maciel, and M. Weinberg, “A Micromachined Comb-Drive Tuning Fork Rate Gyroscope”, Proc. IEEE Micro Electromechanical Systems Workshop (MEMS 93), Fort Lauderdale, Fla., USA, February 1993, pp. 143-148 or DE 19641284. It is possible to design structures such that the resonant frequency of the linear mode is considerably higher than the resonant frequency ω2 of the detection mode. The acceleration-dependent errors described above can therefore be largely suppressed. However, known structures require force transmitters and/or taps for applying forces in the z direction and for measuring movements in the z direction.
In all embodiments disclosed in WO 2005/066585 A1, WO 2005/066584 A1, U.S. Pat. No. 6,705,164 B2, WO 02/16871 A1 and U.S. Pat. No. 6,691,571 B2 having two coupled gyro structures, in which only the two excitation units (and not the sample masses or Coriolis element/detection units) are coupled, the resonant frequencies of the two sample masses and/or of the two Coriolis elements together with the respective detection units are split due to manufacturing tolerances. A large oscillation Q-factor of the detection modes must be achieved for high accuracy in a micromechanical gyro. The resonance (or 3 dB) width of the detection modes can be narrower than the splitting of the two resonant frequencies. For the so-called double-resonant operation required for high accuracies (in which the resonant frequency of the detection mode must be matched to that of the excitation mode), it is then necessary to tune both detection modes individually. In electronic tuning, both detection modes must then be detected and tuned, thus approximately doubling electronics complexity.